The present invention relates to audio-level control systems for stereophonic equipment and more particularly to optically actuated audio-level control systems.
Stereophonic volume control, capable of keeping both channels tracking precisely in level, is extremely difficult. The most widely accepted technique includes straight mechanical control in the form of multi-carbon-mix, multi-tapered-element, multi-fingered-wiper dual potentiometers (pots). Such pots are becoming rather expensive and are not adaptable to remote control, except by the unrefined approach of using a small, remotely actuated DC motor to turn the pot.
Two forms of electrically controlled audio-level devices have been developed for higher-end equipment. The first uses digital switches (field-effect transistors or FETS) to select taps on an attenuator. Although these systems provide accurate channel tracking, they typically have course resolution. Further, these systems are prone to switching transients when moved, creating an audible "zipper noise". Third, interfacing such devices with conventional rotary control has proven difficult.
A second electrically controlled audio-level device includes a voltage-controlled amplifier (VCA) to use the fundamental current-splitting properties of bipolar transistor pairs. These circuits require meticulous device matching, distortion trimming, and thermal environment control. Manufacturing practicalities render such units impractical; and, therefore, audiophiles are often and rightly suspicious of such devices.
Optically actuated audio-level controls have also been designed as illustrated in U.S. Pat. No. 4,700,060 issued Oct. 13, 1987 to Laiacona et al entitled DEVICE FOR SELECTIVELY ROUTING AUDIO SIGNALS BIDIRECTIONALLY ALONG ONE OR TWO SIGNAL PATHS and U.S. Pat. No. 4,434,325 issued Feb. 28, 1984 to Kobayashi et al entitled VOLUME CONTROL FOR AN AUDIO APPARATUS. However, these systems are not suitable to precision level control because they are analogous to a potentiometer with a "loose knob". That is to say that their relation of resistance to LED excitation current is variable with at least production factors and operating temperatures.